First koan (reflective question) in the 13th century Rinzai Zen collection called The Gateless Gate.
A monk asked Zhaozhou Congshen, a Chinese Zen master (known as Jōshū in Japanese), “Has a dog Buddha-nature or not?” Zhaozhou answered, “Wú” (in Japanese, Mu)
Meaning–say I, who never passed even one koan–“Asking whether a dog has the Buddha-nature [the capacity to become enlightened] is an unanswerable question, and so my answer to you is NOTHING.”
Of course, there’s a regional pronunciation, so I’m told, of “Mu” which sounds more like
“WOOO[F]!”
So maybe Jōshū was saying “the dog knows its own nature; stop asking stupid questions.”